The present invention relates to a method for producing a halftone plate having a certain screen angle from an original picture having a continuous tone directly for use in a picture reproducing machine such as a color scanner for plate making and a color facsimile.
In a conventional picture reproducing machine such as a color scanner, a halftone plate is produced from an original picture having a continuous tone by exposing a lithographic recording film via a halftone contact screen overlapped thereon by using a light beam modulated by a picture signal picked up from the original picture by scanning. Alternatively, the original picture and the halftone contact screen are scanned separately in the synchronous relation to obtain output signals and then the output signals are treated electrically to produce a halftone plate having a certain screen angle.
However, in such methods wherein the halftone contact screen is used, there are disadvantages such as high cost, unevenness of halftone dots by a poor contact between the halftone contact screen and the film, a large exposure light source, and an inevitably complicated and large machine.
Other methods wherein the halftone dots are produced electrically, have been developed. For example, in a halftone dot pattern having a screen angle similar to the desired screen angle, the desired halftone dot data to be stored in a memory can be reduced by selecting a screen angle for which a basic halftone structure having the earliest period is produced, as disclosed in Japanese Patent Publication No. 52-49361.
In this method, a screen angle is selected so that its tangent value may be a rational number, and a square halftone dot pattern which is defined by axes extending in the directions of the scanning direction and the moving direction of the head in the rectangular coordinates, is repeated over the basic halftone structures. Hence, a reproduction picture is recorded by using this repeat pattern, and thus the necessary halftone dot data to be stored in the memory can be reduced.
In this embodiment, for example, when a screen angle .alpha. is determined so that its tangent value tan .alpha. equals a rational number y/x, wherein x and y mean integral multiples of a screen line pitch, one square repeat pattern includes the x.sup.2 +y.sup.2 number of entire halftone dots. This means that, in the conventional method, even if the screen angle is determined so that its tangent value may be a rational number, the halftone dot data for one square repeat pattern including at least x.sup.2 +y.sup.2 number of vignette halftone dots is essential for reproducing the original picture.
In this method, however, it is difficult to select a screen angle similar to the desired screen angle, whose tangent value is a rational number, without increasing the necessary halftone dot data. In practice, the screen angle whose tangent value is a rational number so that the desired halftone dot data may be the minimum, i.e. the x.sup.2 +y.sup.2 number of halftone dots, is preferentially selected.
For instance, when the screen angle of 15 degree, which is often selected for the color separation for plate making, is desired, a rational number y/x similar to the value of tan 15.degree. is 1/4, 3/11, 4/15, 5/19, or the like. The vignette halftone dots included in one repeat pattern are so calculated for each rational number: 17 for 1/4, 130 for 3/11, 241 for 4/15, 386 for 5/19, or the like.
Meanwhile, the actual screen angle for the rational number 1/4, 3/11, 4/15, or 5/19 is 14.036, 15.255, 14.931 or 14.743.degree., respectively and hence the repeat pattern for the rational number 4/15, the closest approximation to the screen angle of 15.degree. includes 241 vignette halftone dots.
In order to obtain the desired halftone dot data of the repeat pattern including 241 vignette halftone dots continuously and electrically, the corresponding part of a halftone contact screen is scanned as described above, or the desired halftone dot data stored in a memory in advance, which is operable in real time processing, is read out of the memory consecutively. However, the former method requires a complicated large machine as described above, and the latter method involves a memory having a large capacity. Both methods are not practicable.